I'm currently in middle of AT&T 2-year contract. My phone features 3G connection. And the Media Net Unlimited service(is it imply 3G?) is $15 per month which I have not subscribed yet. Can I buy a iphone 3G from ebay (doesn't have to be unlocked since I'm with AT&T), and use the Media Net Unlimited. It will be $15 saving per month (my plan is 39.99 + $15 data). Anyone see any problem about that? Can AT&T detect my phone and reset my service plan?